Strontium-85
Sr-85 · Strontium, Z = 38, A = 85
At 8.77e+14 Bq/g — 2.37e+4 Ci/g — a gigabecquerel of Sr-85 amounts to 1.14 µg, which is why activity rather than mass is how anyone states the quantity. Strontium-85 decays by electron capture with a half-life of 64.849 days, falling to 72.57% of today's activity in a month and 2.02% in a year.
1 GBq at 1 m reads 0.0669 mGy/h, and 1 Ci at the same distance 2.48 mGy/h, from an air kerma rate constant of 0.0669 mGy·m²/(GBq·h) — 1.2× less than Cs-137 and 4.6× less than Co-60, and 33 of 96 among the photon emitters carried here.
4 lines clear the 20 keV cutoff, but one of them carries 90% of the dose rate. The leading one is 514.0 keV at 100.0% of the total — its emission probability is 95.712%, which is also the highest.
Halving the air kerma rate calls for 3.93 mm of lead, or 10.6 mm of steel where lead is unwelcome, and a factor of ten calls for 13.1 mm of lead — sheet thicknesses that a glovebox or a transport container can carry. Reaching 20 µSv/h from 1 GBq at a metre takes 6.86 mm of lead.
Half-life, specific activity and dose rate
| Half-life | 64.849 days (5.603e+6 s) |
| Decay mode | electron capture |
| Specific activity | 8.77e+14 Bq/g (2.37e+4 Ci/g) |
| Air kerma rate constant Γ (δ = 20 keV) | 0.0669 mGy·m²/(GBq·h) |
| Dose rate, 1 GBq at 1 m | 0.0669 mGy/h |
| Dose rate, 1 Ci at 1 m | 2.48 mGy/h |
| Kerma-weighted mean photon energy | 514.1 keV |
514 keV carries 100% of the dose rate
6 further lines below the 20 keV cutoff, the highest at 15.19 keV and 70.7% emission probability in all, are excluded here and from Γ. Why the two columns rank differently.
| Energy (keV) | Emission probability (%) | Share of dose rate (%) |
|---|---|---|
| 514.00 | 95.712 | 99.98 |
| 868.06 | 0.012 | 0.02 |
| 362.82 | 0.00137 | 0.00 |
| 151.18 | 0.00115 | 0.00 |
3.93 mm of lead halves this spectrum
Solved numerically across all 4 lines, narrow beam. Why not one representative energy.
| Material | HVL (mm) | TVL (mm) | TVL / HVL |
|---|---|---|---|
| lead | 3.93 | 13.1 | 3.32 |
| tungsten | 2.70 | 8.97 | 3.32 |
| iron | 10.6 | 35.2 | 3.32 |
| copper | 9.38 | 31.2 | 3.32 |
| concrete | 34.2 | 114 | 3.32 |
| water | 72.4 | 241 | 3.32 |
| aluminum | 30.8 | 102 | 3.32 |
A single energy would give 3.32. What a spread of energies does instead.
Activity over months and years
Ten half-lives is 1.78 years — a storage problem rather than a disposal one, with 2.02% of today's activity still there after a year. The mean life 1/λ is 93.6 days.
| Elapsed | Fraction remaining |
|---|---|
| 1 half-life | 50.0 % |
| 2 half-lives | 25.0 % |
| 5 half-lives | 3.13 % |
| 10 half-lives | 0.0977 % |
| Time to fall to 10 % of today's activity | 215 days |
| Time to fall to 1 % | 1.18 years |
| Time to fall to 0.1 % | 1.77 years |
Limits of these dose rates
- 0.0669 mGy/h at a metre — bare point source, no capsule, no self-absorption.
- 3.93 mm of lead halves this spectrum, narrow beam, scatter not added back.
- Γ excludes 6 lines under 20 keV, carrying 70.7% of all emissions.
- What every sheet leaves out, internal dose included.
Gamma and decay calculators for Sr-85
Other Strontium nuclides: Sr-85m, Sr-89, Sr-90
Computed from the IAEA Nuclear Data Section — Livechart API (ENSDF) and the NIST X-Ray Mass Attenuation Coefficients. Derivations and citations.